1. Field
A force-measuring cell with a deformable body is disclosed, with at least one strain gauge installed on the deformable body by an adhesive layer, wherein the at least one strain gauge has a strain-sensitive electrical resistor track arranged on a carrier substrate. A method of bonding a strain-gauge to a deformable body is also disclosed.
2. Background Information
A strain gauge has a carrier substrate on which a metallic resistor track is arranged which can be made in the shape of a meandering structure by a known chemical etching method. Also arranged on the carrier substrate are connector electrodes for contacting the resistor track. The connector electrodes are often made in one work operation together with the resistor track, and they can consist therefore of the same material. Electrically insulating materials are used for the carrier substrates of strain gauges. Depending on the area of application, one finds carrier substrates of glass, ceramic materials, in many cases polymers, glass-fiber reinforced polymers, or composite materials. Strain gauges are measuring elements in which a mechanical deformation causes a change of the electrical resistance and which are therefore used for the measurement of the force that produces the deformation.
In the field of weighing technology, to name an example, a force acting on a deformable body causes a deformation which is converted into an electrical signal by strain gauges. In a force-measuring cell that functions according to this principle, a load on the weighing pan which is connected to the vertically movable load-receiving part of the deformable body produces a displacement of the load-receiving part in relation to the spatially fixed part of the deformable body. In an exemplary embodiment, the deformable bodies used in force-measuring cells have four elastic bending zones formed by thin material portions which are located at the four corners of a parallelogram, so that the load-receiving part is arranged as a vertically movable leg of the parallelogram opposite a fixed, likewise vertical parallelogram leg that is preferably fastened to the housing of a weighing scale. The magnitude of the deformation that occurs in the thin bending zones is measured as an electrical resistance change by at least one strain gauge that can be installed on one of the bending zones by an electrically insulating adhesive layer.
Because of their elastic properties, polymer substrate materials are an exemplary choice for strain gauges used in the field of weighing technology, in particular polyimides, but also epoxy resins, phenolic resins, melamines and ketones. Polymer carrier substrates have the advantage of a lower rigidity, so that their shape can conform more easily to the deformable body. This reduces in particular the mechanical stress on the adhesive layer. Hysteresis effects or a destruction of the adhesive layer that can occur when a rigid substrate is bonded to a deformable body are found far less often with polymer substrates. Furthermore, in the case of strain gauges with a meander-patterned resistor track polymer substrates offer the possibility of compensating a drift in the load signal through the known method of designing the return loops of the resistor track with an appropriately selected shape. Besides, strain gauges with polymer carrier substrates are easier to handle and more cost-effective to produce.
Epoxy compounds have been used for the adhesive layer, known for example under the trade designations M-Bond 610 or M-Bond 43-B, which are available from Vishay Micro-Measurements. These adhesives, which are in liquid form, are applied to the deformable body in the areas of the bending zones, for example with a brush at room temperature. Next, the strain gauge is put in place and the adhesive bond is hardened in an oven preferably under pressure and at temperatures between 150° C. and 180° C. The time period for the exposure to the increased temperature is a few hours, as a rule 6 to 8 hours.
The drawbacks associated with the use of this adhesive material and the method of bonding the strain gauge to the deformable body are that on the one hand the deformable body will during the exposure to the increased temperature in the hardening process change its elastic properties towards an increase in the inelastic effects and that on the other hand so-called thermal stresses will build up in an installed strain gauge in particular during the cooling-down process. Dependent on the hardening temperature and also on the subsequent storage temperature this will lead to a relaxation of these stresses, that may remain observable over an extended time period, i.e., up to a few months.
Inorganic-organic hybrid polymers which are known, e.g., under the trade name ORMOCER® are a new class of bonding materials that consist of inorganic and organic networks which connect with each other and penetrate each other on a molecular level. They are produced according to a sol-gel process in the presence of acidic or basic catalysts. They are distinguished by their high durability, high pressure- and scratch resistance, as well as an excellent module of elasticity. In addition, they can be produced cost-effectively. To name an example, the use of inorganic-organic hybrid polymers in the field of dental technology is described in DE 100 16 324 A1.
A method of producing these materials is described in DE 43 03 570 A1. In a first step, a hydrolytic polycondensation of crosslinkable organofunctional silanes and at least one metallic compound takes place. This condensation can in some cases take place in the presence of non-crosslinkable organofunctional silanes and low-volatile oxides that are soluble in the reaction medium. In a second step, an organically crosslinkable pre-polymer is added, followed by a third step in which the material is put on a substrate and a fourth step in which the material is hardened by radiation or heat treatment.
The inorganic-organic hybrid polymers are credited with a high abrasion- and scratch resistance as well as good adhesion to any base materials such as metals, plastics, glass and ceramics. The inorganic network lends properties such as hardness and thermal stability to the inorganic-organic hybrid polymers, while the organic network determines the elastic properties. Physical properties such as, e.g., the modulus of elasticity or the coefficient of thermal expansion are influenced by the ratio between the respective degrees of inorganic and organic crosslinking. The physical properties can also be modified by adding filler materials.
A lamination adhesive of inorganic-organic hybrid polymers is disclosed in DE 101 38 423 A1, where modified filler materials are tied into the inorganic-organic polymer network through covalent or ionic bonds. This can improve the barrier effect against gases and vapors of the compound systems, such as foils for the packaging of food products.
All of the aforementioned documents are hereby incorporated by reference in their entireties.